A goal of many modem long haul optical transport systems is to provide for the efficient transmission of large volumes of voice traffic and data traffic over trans-continental distances at low costs. Various methods of achieving these goals include time division multiplexing (TDM) and wavelength division multiplexing (WDM). In time division multiplexed systems, data streams comprised of short pulses of light are interleaved in the time domain to achieve high spectral efficiency, high data rate transport. In wavelength division multiplexed systems, data streams comprised of short pulses of light of different carrier frequencies, or equivalently wavelength, are co-propagate in the same fiber to achieve high spectral efficiency, high data rate transport.
The transmission medium of these systems is typically optical fiber. In addition there is a transmitter and a receiver. The transmitter typically includes a semiconductor diode laser, and supporting electronics. The laser may be directly modulated with a data train with an advantage of low cost, and a disadvantage of low reach and capacity performance. After binary modulation, a high bit may be transmitted as an optical signal level with more power than the optical signal level in a low bit. Often, the optical signal level in a low bit is engineered to be equal to, or approximately equal to zero. In addition to binary modulation, the data can be transmitted with multiple levels, although in current optical transport systems, a two level binary modulation scheme is predominantly employed.
Often, the modulator is separate from the laser diode. This allows for a carrier signal with higher spectral purity and higher reach and capacity performance. One modulator may be used to directly encode the data onto the laser signal. For example, one modulator may be used to achieve a non-return-to-zero (NRZ) format. In a non-return-to-zero format, the instantaneous power of a high optical signal does not return to the low value between adjacent high data bits.
For best long haul transmission performance, the return-to-zero (RZ) performance is used. RZ signals, however, exhibit a larger bandwidth than NRZ signals. In practice, a two stage modulator may also be used to achieve this improved performance. For example, a first modulator may be used to shape a train of all high optical pulses with good contrast to the low value between pulses. A second modulator may then be used to encode the data onto this stream of pulses, effectively attenuating those bits that are to be encoded as zeros.
During transmission, attenuation in the optical fiber is compensated by in line optical amplifiers. The optical signal propagates through a span of fiber that may be 60-120 km long. At the end of each span, the signal is amplified in an optical amplifier. This process may be repeated over 60 times, for a total system reach of approximately 6000 km. The limit to the number of times this process may be repeated is determined by the optical noise from the optical amplifier.
The receiver is located at the opposite end of the optical fiber, from the transmitter. The receiver is typically comprised of a semiconductor photodetector, electrical amplifier, filter and decision circuitry. The role of the decision circuitry is to determine whether a bit is a zero (low) or a one (high) as accurately as possible in the presence of noise, or uncertainty in the level of the received bit. The resultant electrical noise in the received electrical signal stems from mixing of the optical noise with the signal power. Therefore the amount of noise in the (higher power) one rail is larger than the noise on the zero rail. Typically, the standard distribution of the probability density of ones is three times the standard deviation of the probability density of zeros. The decision process leads to the concept of a minimum or optimum bit-error-rate (BER). Erred ones are transmitted signal ones that are mistakenly detected as zeros. Erred zeros are transmitted signal zeros that are mistakenly detected as ones. The rate at which these errors occur is the BER. Typical BERs for optical transport equipment are in the 10−12 to 10−15 range. A BER of 10−15 implies that one erroneous reading is made in 1015 bits sent.
Current decision circuitry control in the art assumes that the number of erred ones are equal to the number of erred zeros for optimum BER. Since the standard deviations of the probability densities are different, this assumption is sub-optimal, and leads to an unnecessarily high BER. There is a need for a decision apparatus and method that sets the optimal threshold. Further there is a need for a decision apparatus and method that sets the optimal threshold in light of unequal standard deviations in the probability densities.